Electric generators, such as those used in the power generation industry, essentially comprise a rotor and a stator. The rotor is wound with conductors to form a field winding. The stator is wound with conductors to form a stator winding. The field winding is supplied with an excitation current in order to create a magnetic field on the rotor. When the rotor spins inside the stator, electric power is induced in the stator winding.
The rotor of an electric generator is generally machined from a solid steel forging. Slots are provided along the length of the rotor for inserting the conductors that make up the field winding. FIG. 1 illustrates a slot 10 of a typical synchronous generator rotor 12. The slot 10 is filled with multiple copper conductors 14. Each copper conductor is separated from adjacent copper conductors by an insulation layer 16. The conductors 14 extend down the axial length of the rotor 12. A pair of electrically connected conductors 14 is referred to as a rotor turn 18.
When an electric generator is first manufactured, each rotor turn 18 is electrically insulated along the axial length of the rotor from adjacent rotor turns by the insulation layer 16. Over time, various factors related to the normal operation of the generator may cause damage to the insulation layer 16. Damage to the insulation layer 16 may permit adjacent rotor turns to come into electrical contact. This situation is referred to as a rotor turn short. Rotor turn shorts significantly reduce the overall inductance of the field winding of a rotor and impair the efficiency and output capability of a generator. When sufficient rotor turn shorts occur, a rotor must generally be rewound to repair the shorts.
The conventional approach to detecting shorted rotor turns in an operating generator involves the use of a flux probe. A flux probe is basically a coil in which a voltage is induced by a varying magnetic flux. The flux probe is used to measure the magnetic field associated with each rotor pole by placing the probe inside the generator air gap and then observing and comparing the flux associated with each rotor pole. The main problems with flux probes are that the data acquired by flux probes are sensitive to generator load and the interpretation of the data is relatively subjective. Flux probe systems are also quite expensive, generally costing in excess of $25,000 per unit.